1. The purpose of this Core is to provide a facility for the propagation of xenografts of pediatric solid tumors, for use in all Projects. The standard model will comprise CBA/CaJ mice immune-deprived by a procedure of thymectomy, whole body irradiation and marrow reconstitution. For specific experiments, SCID mice will be used to establish new xenograft lines (glioblastoma) or where growth of tumors in the standard model is not optimal (some osteosarcomas). 2. Coordinate tumor transplantation and make available tumor-bearing mice for biochemical and pharmacokinetic studies as required. 3. Undertake drug evaluation studies using standardized protocols. Data will be accessed directly into a microcomputer, and information distributed on a regular basis to individual project leaders and to the Biostatistics Core for analysis. 4. Establish models of minimal residual disease for evaluating both cytotoxic and novel approaches to tumor ieradication (e.g., gene therapy). 5. Evaluate orthotopic implantation as a secondary screen for drug evaluation. 6. Continue to derive models of tumors resistant to camptothecin inhibitors of topoisomerase I. 7. Maintain and characterize human xenografts, and maintain frozen stocks. 8. Provide services (e.g. blood collection, 1 to 5 day infusions in mice). A program "working committee" will serve to prioritize the order for evaluating new agents and strategies derived from individual projects, and for allocating resources.